Apparatus and method for self-testing a component for signal recovery

ABSTRACT

A circuit having a component for signal recovery, such as an adaptive equalizer, may be tested in order to ensure that the component operates properly. Unfortunately, external test equipment may be expensive and prone to being damaged. According to an aspect of the disclosure, there is provided a circuit including BIST (Built-in Self-Test) circuitry for testing a component for signal recovery with a stress signal that simulates an imperfect signal received over a communication channel. The circuit also has a detector for determining whether the component is operating properly with the stress signal. Thus, no external test equipment is needed for testing the component. In some implementations, the BIST circuitry includes a low-pass filter for filtering a transmit signal into the stress signal. Thus, the amount of circuitry involved in generating the stress signal can be reduced.

RELATED APPLICATION

This application is a continuation of previously filed U.S. applicationSer. No. 13/706,887 filed on Dec. 6, 2012, the entire disclosure ofwhich is hereby incorporated by reference.

FIELD OF THE DISCLOSURE

The disclosure relates to communication systems, and more particularlyto recovering data received over a communication channel.

BACKGROUND

In the past, a motivation for deploying multimode fibre was its cheapercost by comparison with single mode fibre. Unfortunately, multimodefibre has disadvantages over single mode fibre. In particular, signalstraversing a multimode fibre will be dispersed or separated into anumber of modes. Unfortunately, depending on the length of the multimodefibre, each mode may arrive at the destination at slightly differenttimes. A receiver should compensate for this. Furthermore, the receivershould compensate for ISI (Inter-Symbol Interference) found in thereceived signal.

Recovering data received over a multimode fibre is accomplished usingappropriate timing recovery. Many existing timing recovery circuits usea PLL (Phase Lock loop) to determine a proper sampling point. However,as data rates increase, existing timing recovery circuits are no longeradequate for properly recovering data received over multimode fibre.Adaptive equalization with appropriate timing recovery circuits can beused. An entire study group during the 802.3aq standard was involved incharacterizing this problem.

Adaptive equalization can be an effective approach for recovering datareceived over multimode fibre. Additionally, adaptive equalization canbe an effective approach for recovering data received over other typesof communication channels such as long connections of a PCB (PrintedCircuit Board) for example.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described with reference tothe attached drawings in which:

FIG. 1 is a block diagram of a circuit with BIST (Built-in Self-Test)circuitry for testing a component used for signal recovery;

FIG. 2 is a block diagram of a circuit with BIST circuitry for testingan adaptive equalizer;

FIG. 3 is a block diagram of a circuit with BIST circuitry for testingan adaptive equalizer having a feed-forward equalizer and a decisionfeedback equalizer;

FIG. 4 is a block diagram of an analog filter;

FIG. 5 is a block diagram of a digital filter;

FIG. 6 is a block diagram of BIST circuitry featuring a filter and anoise generator;

FIG. 7 is a block diagram of BIST circuitry featuring transformationmodules; and

FIG. 8 is a flowchart of a method for testing a component used forsignal recovery.

DETAILED DESCRIPTION OF EMBODIMENTS

It should be understood at the outset that although illustrativeimplementations of one or more embodiments of the present disclosure areprovided below, the disclosed systems and/or methods may be implementedusing any number of techniques, whether currently known or in existence.The disclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated below, includingthe exemplary designs and implementations illustrated and describedherein, but may be modified within the scope of the appended claimsalong with their full scope of equivalents.

A circuit having a component for signal recovery, such as an adaptiveequalizer, may be tested in order to ensure that the component operatesproperly. Such testing may be performed because proper operation issubject to a variety of challenges introduced by analog components. Oneapproach is to use external test equipment to generate an input signalfor the component and to observe how the component reacts. However,external test equipment may be expensive and prone to being damaged in aharsh ATE (Automated Test Equipment) environment. Also, it may bedifficult or impractical to use external test equipment in somesituations such as when the circuit is tested in the field. Note thatthe circuit may be tested in the field to help isolate where a problemmay be occurring in a system and it is not known if the problem is withthe circuit or with some other component in the system.

According to an aspect of the disclosure, there is provided a circuitincluding BIST (Built-in Self-Test) circuitry for testing a componentfor signal recovery with a stress signal that simulates an imperfectsignal received over a communication channel. The circuit also has adetector for determining whether the component is operating properlywith the stress signal. Thus, no external test equipment is needed fortesting the component. In some implementations, the BIST circuitryincludes a low-pass filter for filtering a transmit signal into thestress signal. Thus, the amount of circuitry involved in generating thestress signal can be reduced.

Other aspects and features of the present disclosure will becomeapparent, to those ordinarily skilled in the art, upon review of thefollowing description of the specific embodiments of the invention.

BIST Circuitry for Testing a Component

Referring first to FIG. 1, shown is a block diagram of a circuit 100with BIST circuitry 120 for testing a component 111 used for signalrecovery. The circuit 100 has receive circuitry 110 including thecomponent 111 used for signal recovery, the BIST circuitry 120, and adetector 130. The circuit 100 may have other components, but they arenot shown for sake of simplicity.

The operation of the circuit 100 will now be described by way ofexample. The receive circuitry 110 can receive and process a receivesignal 191. The receive signal 191 might for example be received over acommunication channel (not shown) that introduces at least one defectsuch as ISI (Inter-Symbol Interference). In order to accommodate forthis, the receive circuitry 110 includes the component 111 for signalrecovery.

The circuit 100 might be tested in order to ensure that the component111 operates properly. Unfortunately, as already stated, external testequipment may be expensive and prone to being damaged. According to anaspect of the disclosure, the BIST circuitry 120 tests the component 111with a stress signal 190 that simulates an imperfect signal receivedover a communication channel. Thus, no external test equipment is neededfor testing the component 111. Thus, during testing, there may be lesshardware to be damaged, less hardware to be calibrated, etc. Note thatBIST circuitry 120 is “built-in” in the sense that it permanently formspart of the circuit 100. For example, the BIST circuitry 120 may be onthe same chip (e.g. same silicon) as the rest of the circuit 100, orotherwise permanently fixed.

In some implementations, the circuit 100 has a multiplexer (not shown)for selecting between the receive signal 191 and the stress signal 190.Examples of this are provided below with reference to FIGS. 2 and 3. Inalternative implementations, the receive circuitry 110 receives both thereceive signal 191 and the stress signal 190, but processes only one ofthem based on whether the receive circuitry 110 is in a receive state ora test state. Thus, the multiplexer is not required. Otherimplementations are possible.

The detector 130 determines whether the component 111 is operatingproperly with the stress signal 190. More generally, the detector 130determines at least one condition 192 that is indicative of whether thecomponent 111 is operating properly with the stress signal. There aremany possibilities for such condition 192, some of which will bedescribed in further detail with reference to the subsequent Figures.

There are many possibilities for the component 111 used for signalrecovery. In some implementations, the component 111 is an adaptiveequalizer (e.g. backplane or cable equalizer). Examples of this aredescribed in further detail below with reference to FIGS. 2 and 3. Inalternative implementations, the component 111 is a non-adaptiveequalizer, or a fixed filter designed to compensate for ISI. Suchalternative implementations are not adaptive to varying ISI conditions.

In some implementations in which the circuit 100 includes transmitcircuitry (not shown) for generating a transmit signal, the BISTcircuitry 120 is configured for transforming the transmit signal intothe stress signal 190. In such implementations, the BIST circuitry 120makes use of existing circuitry, namely the transmit circuitry. Thus,the amount of additional circuitry involved in generating the stresssignal 190 can be reduced. The BIST circuitry 120 might for exampletransform the transmit signal into the stress signal 190 using a filterand/or a noise generator. Examples of this are described in furtherdetail below with reference to the subsequent Figures. In alternativeimplementations, the BIST circuitry 120 includes dedicated hardware (notshown) for generating the stress signal 190 from scratch and does notmake use of any transmit circuitry. Other implementations are possible.

More generally, the BIST circuitry 120 can include any appropriatecircuitry to generate the stress signal 190 that simulates an imperfectsignal received over a communication channel. The so-called imperfectsignal is “imperfect” in the sense that it has at least one defect fromtransmission over a communication channel. In some implementations, theat least one defect includes ISI caused by frequency-dependentattenuation of frequencies (e.g. high frequencies) from transmissionover a communication channel. In some implementations, the at least onedefect includes noise as may be introduced from transmission over acommunication channel. Other possible defects may include multipathinterference, channel non-linearity's, reflections, etc. In someimplementations, the at least one defect includes multiple defects suchas both ISI and noise.

BIST Circuitry for Testing an Adaptive Equalizer

Referring now to FIG. 2, shown is a block diagram of a circuit 200 withBIST circuitry 220 for testing an adaptive equalizer 211. The circuit200 has transmit circuitry 240, and receive circuitry 210 including theadaptive equalizer 211 and CDR (Clock and Data Recovery) circuitry 212.The circuit 200 also has the BIST circuitry 220 for testing the adaptiveequalizer 211, a detector 230, and a multiplexer 250. The circuit 200may have other components, but they are not shown for sake ofsimplicity.

The operation of the circuit 200 will now be described by way ofexample. The transmit circuitry 240 can generate a transmit signal 294from data 293. The receive circuitry 210 can receive and process areceive signal 291. The receive signal 291 might for example be receivedover a communication channel (not shown) that introduces at least onedefect such as ISI. In order to accommodate for this, the receivecircuitry 210 includes the adaptive equalizer 211 for signal recovery.The receive circuitry 210 also includes the CDR circuitry 212 forsampling the recovered signal.

The circuit 200 might be tested in order to ensure that the adaptiveequalizer 211 operates properly. Unfortunately, as already stated,external test equipment may be expensive and prone to being damaged orotherwise impractical. According to an aspect of the disclosure, theBIST circuitry 220 tests the adaptive equalizer 211 with a stress signal290 that simulates an imperfect signal received over a communicationchannel. Thus, no external test equipment is needed for testing theadaptive equalizer 211.

The multiplexer 250 provides the stress signal 290 to the receivecircuitry 210 when the stress signal 290 has been selected via a selectsignal 295. This would be done when testing the adaptive equalizer 211(e.g. while the transmit circuitry 240 and the receive circuitry 210 arerunning at probe test). Conversely, the multiplexer 250 provides thereceive signal 291 to the receive circuitry 210 when the receive signal291 has been selected. This would be done when the adaptive equalizer211 is not being tested. Thus, the multiplexer 250 operates to selectone of the two signals 290,291 as an input for the receive circuitry210, depending on whether the adaptive equalizer 211 is being tested.

In alternative implementations, the multiplexer 250 selects anon-stressed signal that originates from somewhere in the transmit path(e.g. the transmit signal 294). However, such alternativeimplementations do not actually stress the adaptive equalizer 211, asthe non-stressed signal does not simulate an imperfect signal receivedover a communication channel. Thus, using a stressed signal is bettersuited for testing the adaptive equalizer 211.

The detector 230 determines whether the adaptive equalizer 211 isoperating properly with the stress signal 290. More generally, thedetector 230 determines at least one condition 292 that is indicative ofwhether the adaptive equalizer 211 is operating properly with the stresssignal 290. There are many possibilities for such condition 192.

In some implementations, the condition 292 is whether recovered datafrom the CDR circuitry 212 matches original data 293 from the transmitsignal 294. A match suggests that the adaptive equalizer 211 isoperating properly, and also suggests that the transmit circuitry 240and the CDR circuitry 212 are operating properly. Thus, indirectly, thetransmit circuitry 240 and the CDR circuitry 212 may also be tested. Nomatch may indicate that the adaptive equalizer 211 is not operatingproperly.

In alternative implementations, the condition 292 is whether theadaptive equalizer 211 has coefficients that are suitable given thestress signal 290. In other words, the condition 292 is whether theadaptive equalizer 211 has adapted in a suitable manner in light of thestress signal 290. Details of the coefficients and how they are adaptedwill be described in further detail with reference to FIG. 3.

In the illustrated example, the BIST circuitry 220 includes a filter 221in a loopback path from the transmit circuitry 240 to the receivecircuitry 210. The filter 221 transforms the transmit signal 294 intothe stress signal 290 via filtering. In this manner, the BIST circuitry220 makes use of existing circuitry, namely the transmit circuitry 240.Thus, the amount of additional circuitry involved in generating thestress signal 290 can be reduced. In some implementations, the BISTcircuitry 220 also includes a noise generator, an example of which willbe described in further detail with reference to FIG. 6. More generally,the BIST circuitry 220 can include any appropriate circuitry to generatethe stress signal 290 that simulates an imperfect signal received over acommunication channel.

There are many possibilities for the filter 221. In someimplementations, the filter 221 is a low-pass filter configured toattenuate high frequencies in the transmit signal 294. In alternativeimplementations, the filter 221 is a high-pass filter configured toattenuate low frequencies in the transmit signal 294, or a band-passfilter configured to attenuate low and high frequencies in the transmitsignal 294. In some implementations, the filter 221 is an analog filter,an example of which will be described in further detail with referenceto FIG. 4. In other implementations, the filter 221 is a digital filter,an example of which will be described in further detail with referenceto FIG. 5. In some implementations, the filter 221 is programmable todifferent configurations for varying the stress signal 290. Otherimplementations are possible.

BIST Circuitry for Testing an Adaptive Equalizer Having FFE & DFE

Referring now to FIG. 3, shown is a block diagram of a circuit 300 withBIST circuitry 350 for testing an adaptive equalizer 311 having afeed-forward equalizer 320 and a decision feedback equalizer 330. Thecircuit 300 has transmit circuitry 360, and receive circuitry includingthe adaptive equalizer 311 and a timing recovery circuit 312. Thecircuit 200 also has the BIST circuitry 350 for testing the adaptiveequalizer 311, a signal processing element 340, and a multiplexer 370.In some implementations, there are signal conditioning components, forexample an AGC (Automatic Gain Control) 336 for adjusting a receivesignal 391 to an appropriate range for the adaptive equalizer 311. Thecircuit 300 may have other components, but they are not shown for sakeof simplicity. It is to be understood that the circuit 300 is shown witha very specific arrangement of components and is for exemplary purposesonly.

The operation of the circuit 300 will now be described by way ofexample. The transmit circuitry 360 has a PLL (Phase locked Loop) andMUX (Multiplexer) 362 for generating a transmit signal 394 from data393. The transmit circuitry 360 also has a driver 361 for amplifying thetransmit signal 394 for transmission. The receive circuitry can receiveand process a receive signal 391. The receive signal 391 might forexample be received over a communication channel (not shown) thatintroduces at least one defect such as ISI. In order to accommodate forthe ISI, the receive circuitry includes the adaptive equalizer 311 forsignal recovery. The receive circuitry also includes the timing recoverycircuit 312 for sampling the recovered signal 397.

The circuit 300 might be tested in order to ensure that the adaptiveequalizer 311 operates properly. Unfortunately, as already stated,external test equipment may be expensive and prone to being damaged orotherwise impractical. According to an aspect of the disclosure, theBIST circuitry 350 tests the adaptive equalizer 311 with a stress signal390 that simulates an imperfect signal received over a communicationchannel. Thus, no external test equipment is needed for testing theadaptive equalizer 311. The multiplexer 370 provides the stress signal390 to the receive circuitry when the stress signal 390 has beenselected via a select signal 295.

The adaptive equalizer 311, the timing recovery circuit 312, and thesignal processing element 344 operate in a similar manner as theadaptive equalizer 211, the CDR circuitry 212, and the detector 230,respectively, as shown in FIG. 2. However, the signal processing element340 is shown to have a DSP (Digital Signal Processor) 341 generatingfeedback 344 in the form of a first set of coefficients c_(i) for thefeed-forward equalizer 320 and a second set of coefficients d_(j) forthe decision feedback equalizer 330. The coefficients c_(i),d_(j) aregenerated based on an error signal 399 such that the adaptive equalizer211 can adapt to minimize or reduce error.

The signal processing element 340 determines whether the adaptiveequalizer 311 is operating properly with the stress signal 390. Moregenerally, the signal processing element 340 determines at least onecondition that is indicative of whether the adaptive equalizer 311 isoperating properly with the stress signal 390. There are manypossibilities for such condition. In some implementations, the conditionis whether recovered data from the timing recovery circuit 312 matchesoriginal data 393 from the transmit signal 394. This might for exampleinvolve digital logic comparing the data 393 to the recovered signal398. Such digital logic may be part of the DSP 341 or separate from theDSP 341 (e.g. dedicated hardware). In alternative implementations, thecondition is whether the adaptive equalizer 311 has coefficientsc_(i),d_(j) that are understood to be suitable (i.e. the adaptiveequalizer 311 has adapted in a manner that is understood to beappropriate given the stress signal 390). This might for example involvecircuitry to check if the coefficients c_(i),d_(j) are within someexpected operating range. Such circuitry may be part of the DSP 341, andmay be controlled via software that looks at the coefficientsc_(i),d_(j) that are computed by the DSP 341 and determines if they arewithin some acceptable range. The software could reside in the DSP 341or form part of some external software program. Other implementationsare possible.

In the illustrated example, the DSP 340 implements an algorithm based onthe LMS (Least Mean Square) algorithm for minimizing or reducing theerror signal 399 by dynamically adjusting the first set of coefficientsand the second set of coefficients. The second set of coefficients areupdated based on past values of the second set of coefficients accordingtod _(j) _(T+1) =d _(j) _(T) +(μ·sign(ε_(T))·sign(data_(T-j))),  [1]where μ is a number between 0 and 1, ε_(T) is the error signal 94, anddata_(T-j) is the sampled recovered signal 398. In the illustratedexample, the threshold element 342 implements the sign function for theerror signal 399 while the timing recovery circuit 312 implements thesign function for the sampled recovered signal 398. The first set ofcoefficients are similarly updated based on past values for the firstset of coefficients according toc _(i) _(T+1) =c _(i) _(T) +(μ·sign(ε_(T))·sign(x _(T-i))),  [2]where x is the receive signal 391. In the illustrated example, thethreshold element 343 implements the sign function for the receivesignal 391.

The equations above use sign functions, which differ from thetraditional LMS algorithm. Implementing sign functions, which have onlytwo possible logical outputs, simplifies computation for the DSP 340.However, in other implementations, other types of DACs (Digital toAnalog Converters) with more than two possible logical outputs may beimplemented so that the DSP 341 may perform computation with manypossible discrete values for signals such as the error signal 399 or thereceive signal 391.

There are many operating conditions for the DSP 341. In specificimplementations, the DSP 341 is parallelized to operate at 1/32 times 10GHz. However, more generally, the DSP 341 may operate under any suitableconditions for computing the first set of coefficients and the secondset of coefficients.

Referring now to the feed-forward equalizer 320, there are three analogdelay elements 321,322,323, four multipliers 324,325,326,327 including afirst multiplier 324 and three additional multipliers 325,326,327, and asummation element 328. The feed-forward equalizer 320 may have othercomponents, but they are not shown for sake of simplicity.

In operation, the three analog delay elements 321,322,323 generate threetime-delayed signals from the receive signal 391. The first multiplier324 generates a first multiplied signal by multiplying the receivesignal 391 with a first coefficient, c₀, of the first set ofcoefficients, c_(i). Each additional multiplier 325,326,327 generates arespective multiplied signal by multiplying a respective one of thetime-delayed signals with a respective one of the first set ofcoefficients, c_(i). The summation element 328 generates thefeed-forward signal 396 by summing the first multiplied signal with eachrespective multiplied signal.

It is to be understood that the feed-forward equalizer 320 is shown witha very specific arrangement of components and that other implementationsare possible. Although three analog delay elements 321,322,323 and fourmultipliers 324,325,326,327 are shown, more generally, any appropriatenumber of analog delay elements and multipliers may be implemented. Forexample, in another implementation, the feed-forward equalizer 320 hasone analog delay element and two multipliers. In other implementations,the feed-forward equalizer is provided with more analog delay elementsand more multipliers than that shown. Other implementations arepossible.

In the illustrated example, each analog delay element 321,322,323 is afractional analog delay element. Accordingly, the feed-forward equalizer320 is a fractional feed-forward equalizer. However, in otherimplementations, the analog delay elements 320 are baud rate or baudperiod delay elements in which case the feed-forward equalizer 320 is abaud rate feed-forward equalizer.

Referring now to the decision feedback equalizer 330, there is a firstsummation element 331, a threshold element 332, two clocked flip-flops334,334, two multipliers 335,336, and a second summation element 337.The decision feedback equalizer 330 may have other components, but theyare not shown for sake of simplicity.

In operation, the first summation element 331 generates a raw recoveredsignal 338 by summing the feed-forward signal 396 with each of twomultiplied signals. The threshold element 332 thresholds the rawrecovered signal 338 thereby generating the gross recovered signal 397.The two clocked flip-flops 333,334 generate two time-delayed signalsfrom the gross recovered signal 397. Each multiplier 335,336 generates arespective one of the two multiplied signals by multiplying a respectiveone of the two time-delayed signals with a respective one of the secondset of coefficients, d_(j). The second summation element 337 generatesthe error signal 399 by summing the raw recovered signal 338 with thegross recovered signal 397. Other implementations of the error signalare possible.

It is to be understood that the decision feedback equalizer 330 is shownwith a very specific arrangement of components and that otherimplementations are possible. Although two clocked flip-flops 333,334and two multipliers 335,336 are shown, more generally, any appropriatenumber of clocked flip-flops and multipliers may be implemented. Forexample, in another implementation, the decision feedback equalizer 330has one clocked flip-flop and one multiplier. In other implementations,the decision feedback equalizer 330 is provided with more clockedflip-flops and more multipliers than that shown. Other implementationsare possible.

Further example details of adaptive equalizers and timing recoverycircuits are provided in U.S. Pat. No. 8,184,686, the entire disclosureof which is incorporated by reference.

Example Implementations of BIST Circuitry

As noted above, there are many possibilities for the BIST circuitry. Ingeneral, any appropriate circuitry that can test a component (e.g.adaptive equalizer) used for signal recovery with a stress signal thatsimulates an imperfect signal received over a communication channel canbe utilized. Example implementations are provided in this section. It isto be understood that such example implementations are very specific forexemplary purposes only. Implementations other than those specificallydepicted in this section are possible and are within the scope of thedisclosure.

In some implementations, as noted above, the BIST circuitry includes ananalog filter. FIG. 4 shows a block diagram of an analog filter 400 forexemplary purposes. The analog filter 400 is a low-pass filterconfigured to attenuate high frequencies from an input 410 to an output420. Such attenuation is accomplished with a resistor 430 and acapacitor 450, and is configurable by selectively enabling or disablingadditional resistors 431,432 and capacitors 451,452 via respectiveswitches 441,432; 461,462. Enabling more resistors 431,432 has theeffect of reducing net resistance, while enabling more capacitors451,452 has the effect of increasing net capacitance. Thus, the analogfilter 400 can be configured to have a desired cut-off frequency of

$\begin{matrix}{{f_{c} = \frac{1}{2{\pi \cdot {RC}}}},} & \lbrack 3\rbrack\end{matrix}$where R is the net resistance and C is the net capacitance. In someimplementations, the cut-off frequency is programmable, for example byprogramming whether the switches 441,432; 461,462 are open or closed.

In other implementations, as noted above, the BIST circuitry includes adigital filter. FIG. 5 shows a block diagram of a digital filter 500 forexemplary purposes. The digital filter 500 can be configured as alow-pass filter that attenuates high frequencies from an input 510 to anoutput 520, or as some other transfer function. The digital filter 500has delay elements 530,531 and multipliers 540,541,542, and isconfigurable by selectively setting coefficients e_(o),e₁,e₂ for themultipliers 540,541,542. A summation unit 550 is coupled to themultipliers 540,541,542 to produce a weighted sum depending on thecoefficients e_(o),e₁,e₂. In some implementations, the coefficientse_(o),e₁,e₂ are programmable.

In some implementations, the digital filter 500 is configured to emulatea communication channel (e.g. multimode fibre). This may involvefrequency-dependent attenuation of frequencies (e.g. high frequencies).For such implementations, the delay elements 530,531 do not need to bebaud rate delays, as they may instead be fractional delay elements. Notethat a potential benefit of digital filters is that they may be lessvariable than analog filters. Thus, the digital filter 500 of FIG. 5 maybe better suited for emulating a communication channel than the analogfilter 400 of FIG. 4.

In some implementations, as noted above, the BIST circuitry alsoincludes a noise generator. FIG. 6 shows is a block diagram of BISTcircuitry 600 featuring a filter 630 and a noise generator 620 forexemplary purposes. The filter 630 functions to generate a stress signal690 by filtering a transmit signal 694 from transmit circuitry (notshown) as similarly described earlier. The filter 630 can be anysuitable filter, examples of which have already been provided and arenot repeated here. The BIST circuitry 600 includes a summation unit 610that functions to introduce noise from the noise generator 620 into thestress signal 690. Thus, the transmit signal 694 is transformed into thestress signal 690 by filtering the transmit signal 694 and byintroducing noise.

There are many possibilities for the noise generator. In someimplementations, the noise generator is a PRBS (Pseudo-Random BinarySequence) generator. In some implementations, the noise generatorincludes a resistor and an amplifier. Other implementations arepossible. More generally, any suitable noise generator can be employed.In some implementations, the noise generator 620 introduces enough noiseto reduce SNR (Signal to Noise Ratio) to a predefined level. In someimplementations, the noise generator 620 is programmable for varyingnoise in the stress signal 690. Thus, the stress signal 690 can bevaried in order to test the component (e.g. adaptive equalizer) used forsignal recovery in a variety of ways.

A combination of filtering and noise introduction can help to bettersimulate an imperfect signal received over a communication channel.However, in alternative implementations, the BIST circuitry has a noisegenerator and no filter. There may be some scenarios in which it isdesirable to have only a noise generator, and other situations in whichit is desirable to have only a filter. Also, there may be some scenariosin which it is desirable to have both a filter and a noise generator.Since there are many possibilities for the filter and the noisegenerator, there are numerous possibilities for the transformation fromthe transmit signal to the stress signal.

In some implementations, the BIST circuitry includes transformationmodules in order to cover different possibilities for the transformationfrom the transmit signal to the stress signal. FIG. 7 shows a blockdiagram of BIST circuitry 700 featuring transformation modules710,720,730 for exemplary purposes. In some implementations, eachtransformation module 710,720,730 includes a filter and/or a noisegenerator. More generally, each transformation module 710,720,730 hasany appropriate circuitry for transforming a transmit signal 794 into astress signal 790 that simulates an imperfect signal received over acommunication channel. The BIST circuitry 700 also has a selector 740for selecting one of the transformation modules 710,720,730 to transformthe transmit signal 794 into the stress signal 790. The transformationmodules 710,720,730 can vary in configuration thereby allowing differentpossibilities for the stress signal 790 even when the same transmitsignal 794 is used.

Method for Testing a Component

Referring now to FIG. 8, shown is a flowchart of a method for testing acomponent used for signal recovery. This method may be implemented by acircuit, for example by the circuit 100 shown in FIG. 1, the circuit 200shown in FIG. 2, or the circuit 300 shown in FIG. 3. More generally,this method may be implemented by any appropriately configured circuithaving receive circuitry with a component (e.g. adaptive equalizer) forsignal recovery, and BIST circuitry for testing the component.

At step 8-1, the BIST circuitry generates a stress signal that simulatesan imperfect signal received over a communication channel. In someimplementations in which the circuit includes transmit circuitry forgenerating a transmit signal, the BIST circuitry is configured fortransforming (e.g. filtering) the transmit signal into the stresssignal. Examples of how this might be accomplished as well asalternative implementations have already been provided and are notrepeated here.

At step 8-2, the BIST circuitry tests the component with the stresssignal. Finally, at step 8-3, the circuit determines at least onecondition that is indicative of whether the component is operatingproperly with the stress signal. In some implementations, the conditionis whether recovered data matches original data from the transmitsignal. Examples of how this might be accomplished as well asalternative implementations have already been provided and are notrepeated here.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced otherwise than as specifically described herein.

I claim:
 1. A method comprising: providing a receive signal to receivecircuitry using a multiplexer when the receive signal has been selected;performing signal recovery using a component of the receive circuitry;generating, using BIST (Built-in Self-Test) circuitry, a stress signalthat simulates an imperfect signal received over a communicationchannel; providing the stress signal to the receive circuitry using themultiplexer when the stress signal has been selected in order to testthe component; and determining, using a detector, at least one conditionindicative of whether the component is operating properly with thestress signal.
 2. The method of claim 1, further comprising: generatinga transmit signal using transmit circuitry; wherein generating thestress signal comprises the BIST circuitry transforming the transmitsignal into the stress signal.
 3. The method of claim 2, wherein thecomponent of the receive circuitry is an adaptive equalizer.
 4. Themethod of claim 3, further comprising: recovering data following thesignal recovery using CDR (Clock and Data Recovery) circuitry; whereinthe at least one condition indicative of whether the adaptive equalizeris operating properly comprises whether recovered data from the CDRcircuitry matches original data from the transmit signal.
 5. A methodcomprising: providing signal recovery using a component of receivecircuitry; testing the component using a stress signal generated by BIST(Built-in Self-Test) circuitry to simulate an imperfect signal receivedover a communication channel; and determining, using a detector, atleast one condition indicative of whether the component is operatingproperly with the stress signal; wherein generating the stress signalcomprises: generating a transmit signal using transmit circuitryprovided in the circuit; and transforming, using the BIST circuitry, thetransmit signal into the stress signal by filtering the transmit signalwith a filter provided in the BIST circuitry.
 6. The method of claim 5,wherein the filter is a low-pass filter configured to attenuate highfrequencies in the transmit signal.
 7. The method of claim 5, whereinthe filter is an analog filter.
 8. The method of claim 5, wherein thefilter is a digital filter.
 9. The method of claim 5, wherein the filteris programmable to different configurations for varying the stresssignal.
 10. The method of claim 5, wherein the component of the receivecircuitry is a non-adaptive equalizer or a fixed filter designed tocompensate for ISI (Inter-Symbol Interference).
 11. A method comprising:performing signal recovery using a component of receive circuitry;testing the component using a stress signal generated by BIST (Built-inSelf-Test) circuitry to simulate an imperfect signal received over acommunication channel; and determining, using a detector, at least onecondition indicative of whether the component is operating properly withthe stress signal; wherein generating the stress signal comprises:generating a transmit signal using transmit circuitry provided in thecircuit; and transforming, using the BIST circuitry, the transmit signalinto the stress signal by introducing noise into the transmit signalwith a noise generator provided in the BIST circuitry.
 12. The method ofclaim 11, wherein: the noise generator is configured to reduce SNR(Signal to Noise Ratio) to a predefined level.
 13. The method of claim11, wherein: the noise generator is programmable for varying noise inthe stress signal.
 14. The method of claim 11, wherein the component ofthe receive circuitry is a non-adaptive equalizer or a fixed filterdesigned to compensate for ISI (Inter-Symbol Interference).
 15. A methodcomprising: performing signal recovery using a component of receivecircuitry comprising a component configured for signal recovery; testingthe component with a stress signal generated by BIST (Built-inSelf-Test) circuitry to simulate an imperfect signal received over acommunication channel; and determining, using a detector, at least onecondition indicative of whether the component is operating properly withthe stress signal; wherein generating the stress signal comprises:generating a transmit signal using transmit circuitry provided in thecircuit; selecting one of a plurality of transformation modules togenerate the stress signal for the component; and transforming, usingthe transformation module that has been selected, the transmit signalinto the stress signal.
 16. The method of claim 15, wherein eachtransformation module comprises at least one of (i) a filter and (ii) anoise generator.
 17. The method of claim 15, wherein the transformationmodules vary in configuration thereby allowing different possibilitiesfor the stress signal.
 18. The method of claim 15, wherein the componentof the receive circuitry is a non-adaptive equalizer or a fixed filterdesigned to compensate for ISI (Inter-Symbol Interference).
 19. Acircuit comprising: receive circuitry comprising a component configuredfor signal recovery; transmit circuitry configured for generating atransmit signal; BIST (Built-in Self-Test) circuitry configured fortesting the component with a stress signal that is generated to simulatean imperfect signal received over a communication channel, the BISTcircuitry being configured to transform the transmit signal into thestress signal by filtering the transmit signal; and a detectorconfigured for determining at least one condition indicative of whetherthe component is operating properly with the stress signal.
 20. Thecircuit of claim 19, wherein the BIST circuitry is configured forfiltering the transmit signal by attenuating high frequencies in thetransmit signal.